The invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device with an optical compensation film.
Narrow viewing angle and low contrast ratio are provided by conventional liquid crystal display (LCD) devices. Different brightness levels occur when the display is viewed at different viewing angles. Brighter images are visible directly in front of the monitor but darker images from the sides. The disparity increases with LCD size.
Among wide-view technologies, optical compensation film is popular, since the wide angle of visibility is achieved with no change to the fabrication process, maintaining low costs. Good optical compensation can be achieved by effectively controlling thickness, birefringence, and transmittance of the optical compensation film.
Different LCD devices use different optical compensation film types, uniaxial and biaxial. Uniaxial optical compensation film can further be divided into A-plate and C-plate types. The refractive index of an A-plate optical compensation film obeys the formula ny=nz≠nx (nx indicates refractive index in x-axis, ny and nz can be reasoned by analogy), with x-axis as its optical axis, parallel to a surface of a compensation film. The refractive index of a C-plate optical compensation film obeys the formula ny=nx≠nz, with z-axis as its optical axis, perpendicular to a surface of a compensation film. The refractive index of a biaxial optical compensation film obeys the formula nx≠ny≠nz. A-plate optical compensation film can be obtained by stretching a polymer film in a single direction while negative C-plate and biaxial optical compensation film can be obtained by stretching a polymer film in one direction, then in another direction, but fabrication utilizing bi-direction stretching is difficult.
Negative C-plate optical compensation film provides negative birefringent properties using rigid polyimide or discotic liquid crystal coated on a substrate. This technique, as disclosed by Stephen Z. D. Cheng and Frank W. Harris of the University of Akron utilizes polyimide with aromatic structures as compensation film materials. However, the polyimide absorbs visible light, darkening the compensation film.
Optical compensation film with discotic liquid crystal was developed by Fuji Photo Film. Discotic liquid crystals are also negative C-plate liquid crystals, which compensate positive C-plate liquid crystal in an LCD device to provide a wide viewing angle. However, discotic liquid crystals are expensive and difficult to prepare.
U.S. Pat. Nos. 5,344,916 and 5,480,964 provide polyimide with aromatic structures. The polyimide with birefringent property is synthesized by diamine and dianhydride, both comprising benzene rings.
U.S. Pat. No. 5,344,916 discloses a polyimide with birefringent properties, synthesized by diamine and dianhydride, both comprising benzene rings. However, the polyimide is yellow or orange, unsuitable for applications requiring an optical compensation film to be transparent or colorless. Further, some monomers of this patent are too expensive for mass production.
U.S. Pat. No. 5,480,964 discloses a polyimide with birefringent properties, synthesized by diamine and dianhydride, both comprising biphenyl. However, this compensation film of this polyimide is also colored, unsuitable for applications requiring an optical compensation film to be transparent or colorless. Further, low solubility is produced, and some monomers thereof are not easy to synthesize, increasing costs of mass production.